Free-cutting stainless steel



nited ates Pate 1 Claim. (Cl. 75-124) This invention relates to free-cutting stainless steels displaying a high degree of machinability.

The principal object of the present invention is to provide free-cutting stainless steels that show enhanced adaptability for machining without sacrifice in corrosionresisting qualities by including appropriate quantities of copper and aluminum as alloying additions in the steels which contain carbon up to about 0.20%, silicon up to about 1.5 manganese up to about 2.0%, chromium from about 11.0% to about 20.0%, at least one element selected from the group comprising phosphorus, sulphur, and selenium from about 0.04% to' about 0.50%, and at least one alloying element selected from the group comprising zirconium and molybdenum up to about 1.0%.

Other objects and advantages will be obvious during the course of the following description.

The present invention accordingly resides in the combination of elements, composition of materials and in the relation of each of the same with one or more of the others, the scope of the application of all of which is more fully set forth in the claim at the end of this specification.

In the prior art chromium-bearing stainless steels have displayed a high degree of either corrosion-resisting qualities or stability in the presence of corrosive atmospheres, water vapor, oxyacids, and various kinds of organic substance. As compared with austenitic stainless steels, this series of alloy have proved more hard to produce either intercrystelline corrosion and or stress corrosion crack, and showed great resistance to sulphur. Besides their prices have been low. Accordingly they have been of universal use in the art. Lacking, however, an appreciable degree of plasticity and'machinability in terms of cutting, grinding, etc., they have also proved peculiarly unsuitable for, e.g., machining in industrial full-scale by the use of automatic machinery.

As conducive to a better understanding of the present invention it may be noted that where great importance is attached to machinability as mentioned above, free-cutting stainless steels have'been in practical usewhich should have contained, at the sacrifice in a certain extent of corrosion-resisting qualities, at least one element selected from the group consisting of phosphorus, sulphur, and selenium, including also molybdenum or zirconium as alloying ingredients to prevent detriment of red shortness due to formation of iron sulphide.

However, to meet ever increasingly strict requirements for machinability consistent with the use of automatic machinery, conventional free-cutting stainless steels confined in the range of the compositions as mentioned in the foregoing have proved far from satisfactory in machining characteristic in terms of cutting or grinding. This is particularly true in the case of complicated machining operation such as threading because of frequent occurrences of pulling-01f or pluck on the finished surfaces of the article subjected to the working. Thus a great deal of ditficulties are encountered in acquiring clean finished surfaces.

The free-cutting stainless steels attending upon the present invention possess highly satisfactory free-machining qualities which overcome the detriment confronting in the art. By illustration, a typical but non-limitative analysis of alloy responding to the present invention includes carbon 0.10%, silicon 0.6%, manganese 0.9%, chromium 16.0%, at least one element selected from the group comprising phosphorus, sulphur, and selenium 0.26%, at least one alloying ingredient selected from the group comprising zirconium and molybdenum 0.7%, copper 1.2%, aluminum 2.0%, and the remainder'substantially all iron. Certain advantages are had even where broader ranges-of the ingredients are employed.

With further reference to the compositions of the freecutting stainless steels according to the present invention, I have found that carbon content should be limited up to about 0.2% because carbon would combine with other elements which are in co-existence, especially with chromium, to form carbide which tends to reduce corrosion-resisting qualities exercising unfavorable effects on machinability. Silicon content should be limited up to about 1.5% because silicon contained in high-chromium.

steels improves either corrosion-resisting qualities or heatresisting qualities but impairs machinability under elevated temperature conditions. Manganese should be limited up to about 2.0% taking consideration of the fact that manganese tends to lessen the corrosion-resisting qualities of the steel, although granular manganese sulphide and manganese-selenium compound, the formation of which are brought forth where sulphur and selenium contents are high enough as in the steels of the present inventlon and which act as tip breakers, not only play a major role in contributing free-cutting characteristic but as well alleviate red-short condition dueto sulphur. l An addition.

of chromium above. 11% to the steel confers corrosion resistance in the presence of atmosphere, thus the steel becomes almost insoluble even with 10% nitric acid. Further addition of chromium, however, lessens machinability while various kinds of corrosion-resisting qualities are improved. Thus the maximum of chromium content is limited up to about 20% in order to secure widest possible range of practical use as stainless steel. Wherecontained in stainless steels, phosphorus possesses no advantageous effects on corrosion resistance. However, since phosphorus that exists as solid solution in ferriticstructure improves cutting characteristic, phosphorus content may be raised above 0.04%, and up to about 0.50% which seems sufficient for conferring adequate cutting characteristic. Further additionof phosphorus, however, would involve offsetting disadvantages. selenium existing in high-chromium steel combine with manganese and molybdenum which are in co-existence in the steelto form sulphide and or selenium compound which impart machining characteristic to the steel, the content of these elements is restricted within a limit ranging from about 0.04% to about 0.50% because the. addition over 0.5% would impair hot working characteristic. Elements zirconium and molybdenum cause ferritic structure stable and also render co-existing sulphur stable by forming zirconium and or molybdenum sulphide which would help to enhance cutting characteristic and prevent red shortness. But an addition of these elements up to about 1% is adequate. Copper as alloying element improves corrosion resisting qualities of stainless steels which are exposed to plain Water, brine, and acids having weak oxidizing function, and enhances the cutting characteristic of ferritic stainless steels. The suitable amount of addition of this element seems to range from about 0.5% to about 3.0% where consideration is taken of red shortness. Aluminum is the major element which produces ferrite, facilitates grain-growth, improves cutting characteristic by causing steel to be frangible, and'imparts peculiar metallic luster to steel. However its content is While sulphur and restricted in a range from about 0.5% to about 3.0% from manufacturing and machining view points.

Based on the reasons as described in the foregoing, the most important feature of the present invention resides in producing solid solution of copper and aluminum in ferritic structure by adding these elements to the steel in order to form in part markedly frangible layers in the steel which impart excellent free-cutting characteristic which eifectively fulfills all requirements encountered in practice. This is certainly in sharp and surprising contrast with the conventional free-cutting stainless steels such as, e.g., A181 430 F. The superiority in the qualities of my steels is supported by the various data set forth in the following. Table 1 is to compare A181 430 F with an embodiment of the free-cutting stainless steels responding to the present invention in chemical analyses, various mechanical properties, and corrosion resistance.

TABLE I (2) SUBJECTED TO GOLD-DRAWING AFTER ANNEALING, REDUCTION IN AREA 7% 61. 4 My steel 6 (3) SUBJECTED TO COLD-DRAWING AFTER ANNEALING,

REDUCTION IN AREA 35% A151 430 F 86. 8 2. 4 272 My steel 98. 3 1. 7 306 (c) Corrosion test [Boiling 40% (wt.) nitric acid solution used] Reduction in weight due to corrosion (el -U A181 430 0. so My steel. 0.38

Cutting characteristic is in general evaluated either from the cutting speed in a certain length of time or the quantity of cutting work at a certain cutting speed, the flowing conditions of the shavings produced, and the external appearance of the finished surfaces of the article subjected to cutting. In this respect AISI 430 F steel is compared with the steel according to my invention as follows:

TABLE II (a) Drilling test [Number of revolutions: 1700-1800 r.p.m.]

Time required for drilling, sec.

AISI 403 F 7-10 My steel 5- 6 (b) T arning and threading test with automatic machinery Turning Threading Rotation per min 2, 000 200 Performance A181 403 F Poor Poor Poor. My steel Good Good Good.

All the foregoing, as well as many other highly practical advantages, attend upon the practice of the present invention. Namely, the free-cutting stainless steels according to the present invention proves in comparison with A181 403 F steel, e.g., which is among the free-cutting stainless steels in practical use very much improved qualities in terms of machining easiness, frangibility of shavings, clean finished surfaces of the articles subjected to machining operation.

What I claim as my invention:

Free-cutting stainless steel consisting essentially of from about 0.03% to about 0.2% carbon, from about 0.1% to about 1.5% silicon, from about 0.1% to about 2.0% manganese, from about 11.0% to about 20.0% chromium, at least one element selected from the group consisting of phosphorus, sulphur, and selenium in an amount of from about 0.04% to about 0.50%, at least one alloying ingredient selected from the group consisting of zirconium and molybdenum in an amount of from about 0.05% to about 1.0%, from about 0.5 to about 3.0% copper, from about 0.5% to about 3.0% aluminum, and a balance of substantially all iron.

References Cited in the file of this patent UNITED STATES PATENTS 

